The present invention relates to thin-film resistor systems of the type having a low temperature coefficient of resistance and, more particularly, to systems adapted for microcircuitry applications.
As already indicated, the term `TCR` presently is employed to connote the well-known change in the resistance value of a resistor material which accompanies unit changes in temperature. There, of course, have been numerous attempts to provide low TCR resistors and the variety of thin-film materials and combinations of materials have been suggested. For example, systems utilizing thin-film cermet films or a combination of a cermet film and a conductive plastic with positive and negative TCR have been proposed. Without comment on the effectiveness of these or other suggestions, a common difficulty of these prior systems is that, in order to achieve the low TCR capability, very special process controls must be applied during their fabrication. In other words, their processing must be performed carefully and with considerable precision if the desired results are to be achieved and, of course, any failure in this regard tends to degrade the reliability and effectiveness of the end product. Of perhaps greater importance, the extensive processing requirements do not permit mass production techniques which, of course, are highly desirable if the products are to be made available at a reasonable cost. A further related difficulty is that the suggested resistor materials either are not readily available or, in the form in which they are available, must be altered considerably before use.
Therefore, one of the principal objects of the present invention is to provide a thin-film, low TCR, bi-film resistor system in which the material used for the bi-film resistors are commercially available resistor materials having nominal properties which, when combined, can provide a system having a TCR no greater than .+-. 50 ppm/.degree. C.
Another object is to provide a thin-film resistor system capable of providing a zero TCR with minimal process controls of the materials used in the system.
Yet another object is to provide a low TCR system that is suitable, both in size and in its electrical properties, for use in microcircuitry applications.
A more specific object is to provide a bi-film resistor system for microcircuitry applications utilizing a combination of a tantalum-based system having a negative TCR with a nickel chromium film having a positive TCR.
Still another important object is to provide a low TCR system which uses the NiCr but in which the NiCr is fully protected against anodic attack.
Other objects and their attendant advantages will become more apparent in the ensuing detailed description.